1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a zoom lens having a large aperture in order of an F-number of 1.7 at the wide angle end, a wide image angle (image angle 2.omega. at the wide angle end of 58.degree. to 70.degree.), a large zooming ratio of 12 to 35 and satisfactory optical characteristics over the entire zoom range, through effective use of aspherical surface in a part of the lens system, adapted for use in a television camera, a phototaking camera, a video camera or the like.
2. Related Background Art
For use in the television camera, phototaking camera, video camera or the like, there has been desired a zoom lens, having a large aperture, a high zoom ratio and highly optical characteristics.
In addition, operability and mobility are important factors particularly in the color television camera for broadcasting purpose, and, responding to such requirements, the image pickup device has become principally composed of small CCD (solid-state image pickup device) such as of 2/3 inch or 1/2 inch.
Since such a CCD has substantially uniform resolving power over the entire image pickup area, the zoom lens to be used in combination is required to have substantially uniform resolving power from the center to the peripheral part of the image area.
There are required high optical characteristics with satisfactory correction of various aberrations such as astigmatism, distortion and magnificational chromatic aberration. There are also required a compact dimension and a light weight with a large aperture, a wide image angle, a high zoom ratio, and a long back focus in order to dispose a color separating optical system or various kinds of filters in front of the image pickup means.
The so-called 4-unit zoom lens, consisting, in the order from the object side, of a first focusing lens unit (focusing group) of a positive refractive power, a second zooming lens unit of a negative refractive power, a third lens unit of a positive or negative refractive power for correcting the image plane varying with the zooming operation, and a fourth imaging lens unit of a positive refractive power, is widely employed as the zoom lens for the broadcasting color television cameras, since such zoom lens configuration can easily achieve a relatively high zoom ratio and a large aperture.
Among such 4-unit zoom lenses, a configuration capable of providing a large aperture of an F-number in the range of about 1.6 to 1.9 and a high zoom ratio of about 13 is proposed for example in the Japanese Patent Laid-Open Application No. 54-127322.
In order to achieve a large aperture (F-number of 1.7 to 1.8), a high zoom ratio (zoom ratio of 12 to 35), a wide image angle (wide angle end image angle 2.omega.=58.degree. to 70.degree.) and satisfactory optical characteristics over the entire zooming range in the zoom lens, it is necessary to properly select the refractive power of each lens unit and the constitution lenses.
For example, in order to obtain high optical performance with little variation of the aberrations over the entire zooming range, it is generally necessary to increase the number of lenses in each lens unit, thereby increasing the freedom in correcting the aberration.
For this reason, in the zoom lens of a large aperture, a wide image angle and a high zoom ratio, a number of lenses inevitably becomes larger to increase the dimension of the entire lens system whereby it becomes impossible to meet the requirements of compactness and light weight.
In relation to the imaging performance, there is at first considered the variation of so-called best image plane, at the center of the image area where the image contrast is highest, by the zooming operation. Such variation results mainly from variation of the spherical aberration corresponding to the zooming operation. Such spherical aberration, causing an influence by the cube of the aperture in the region of the third-order aberration coefficient, is the largest difficulty in achieving a large aperture.
In general, for a zoom ratio Z and a focal length fw at the wide angle end, the variation of the spherical aberration caused by the zooming operation assumes, as shown in FIG. 29, an under (minus) side relative to the Gauss image plane from the wide angle end where the spherical aberration is zero to about a zoom position fm=fw.times.Z.sup.1/4, but such tendency becomes less beyond such zoom position. Then the variation becomes zero at a certain zoom position, and then assumes an over (plus) side.
As explained above, fw indicates the focal length at the wide angle end, and Z indicates the zoom ratio.
This over (plus) tendency becomes strongest in the vicinity of a zoom position fd=(Fno.w/Fno.t).times.ft corresponding to the starting point of so-called F drop where the F-number starts to increase (the lens system starting to get darker), but decreases toward the telephoto end beyond this zoom position and becomes approximately zero at the telephoto end.
In the foregoing, Fno.w and Fno.t are respectively the F-numbers at the wide angle end and at the telephoto end, and ft is the focal length at the telephoto end.
Consequently it is extremely difficult to control the spherical aberration at the telephoto side, particularly in a zoom lens having the start position of F-drop.
Then in expanding the image angle of the zoom lens, the distortion aberration poses the largest difficulty in the imaging performance, since the distortion aberration influences by the cube on the image angle in the area of third-order aberration coefficient.
As shown in FIG. 30, the distortion aberration is considerably under (minus) at the wide angle end (focal length fw). It then gradually increases to the over (plus) side from the wide angle end fw toward the telephoto side (focal length ft). Then, after passing through a position where the distortion aberration is zero, such over tendency becomes largest in the vicinity of a zoom position fm=fw.times.Z.sup.1/4. Such over tendency gradually decreases from the focal length fm to the telephoto end ft. Since this behavior becomes larger with the increase of the image angle at the wide angle end, the distortion aberration becomes extremely difficult to control at the wide angle side in increasing the image angle of the zoom lens.
The satisfactory correction of such various aberrations over the entire zooming range has been achieved by increasing the number of lenses in the focusing lens unit or in the zooming lens unit. For this reason the entire lens system has become large and complex.
In order to solve such drawbacks the introduction of an aspherical surface is proposed for example in the Japanese Patent Laid-Open Application No. 8-184758.
However in the recent zoom lenses of improved specifications with a large aperture and a high zoom ratio starting from a super wide image angle, it is becoming necessary to restudy the method of use of the aspherical surface.
In a zoom lens of a large aperture with a high zoom ratio starting from an ultra wide image angle, the spherical aberration varies greatly at the telephoto side while the distortion aberration varies greatly at the wide angle side. In order to satisfactorily correct both aberrations, it is necessary to introduce an aspherical surface in an appropriate position of the zooming part.